Chemical structure and biological activity 



conspicuous intrinsic auxin activity, while the affinity to the growth centres 

 remains fairly high. The anti-auxin effects of /ra/zj'cinnamic acid (Tr) was 

 first demonstrated by Overbeek et al. (1951) in experiments with pea stem 

 sections. They have been confirmed here in experiments with flax roots 

 {Figure 3). The peculiar form of the action curve of the pure substance (a 

 'plateau' at G = 90 in the range 3x10"^ to SxlQ-'^M) does, however, 

 indicate that complications, in the form of a slight synergistic activity, may 

 perhaps occur. In the wheat root test {Figure 3) we found a conspicuous 

 stimulation preceded by faint and uncertain signs of an inhibition at lower 

 concentrations which might again be related to the possible 'synergistic' 

 component of its activity; such a synergistic effect might easily become 

 wholly concealed by the stronger anti-auxin effects in this test. The 

 weak action of Tr in relation to 2:4-D introduces some doubt to such a 

 hypothesis (cf. below), but the synergistic phenomena are not fully under- 

 stood and may comprise mechanisms of different types (see Aberg and 

 Jonsson, 1955). 



In 4-chloro-ira«5cinnamic acid (4-ClTr) the anti-auxin activity is increased 

 by nearly one hundred times in relation to Tr as judged from the effects on 

 wheat roots and on 2:4-D-inhibited flax roots {Figure 3). It will be an interest- 

 ing problem in further studies to decide if the stronger para chlorine effect 

 observed in this instance, as compared to that shown by the pair 

 POzB-4-ClPOzB, may be related to an elimination of the possible 'synergistic' 

 component of the activity of Tr, or whether in fact it is due to an appreciable 

 difference in the effect upon affinity. With 2-chloro-/rawj'cinnamic acid 

 (2-ClTr) , the optimum stimulating concentration for wheat roots is 3 X 1 0~^ M, 

 as compared to 3 X 10~^ M in the case of Tr. This may be due to increased 

 toxicity (or the induction of some faint intrinsic auxin activity) as a result of 

 ori^o-chlorination, again perhaps in combination with the elimination of the 

 hypothetical synergistic component in the activity of the unsubstituted acid. 



Para-chlorination of 2-ClTr to give 2:4-dichloro-^ra«j^cinnamic acid 

 (2:4-Cl2Tr) strongly increases anti-auxin activity {Figure ■^), though it does 

 not exceed that shown by 4-ClTr. The absence of any strong effect of 

 ori^o-chlorination on anti-auxin activity is in good agreement with the 

 conditions in the isohutyv'ic acid series described by Burstrom (1951a). 

 It is interesting to note that the increase in 'toxicity' from Tr to 2-ClTr 

 strongly diminishes the apparent anti-auxin effects of the latter substance. 

 Such a phenomenon well illustrates the unreliability of judging anti-auxin 

 effects from one test or one set of conditions only. 



Both in the pair PO/B-4-ClPO?B and in Tr-4-ClTr, the anti-auxin effects 

 on wheat roots become perceptible at considerably lower concentrations 

 than those producing similar effects on 2:4-D-inhibited flax roots, which is 

 only to be expected when one considers the higher total auxin concentration 

 in the last case. With benzoic acid (phenyl-formic acid, PF), however, we 

 meet an instance where the strength of the effects is reversed. The compara- 

 tively slight stimulation of wheat root growth {Figure 4) has its counterpart in 

 the much lower restorative effect exerted upon lAA-inhibited flax roots 

 (unpublished experiments) than upon the 2:4-D-inhibited ones. The action 

 curve of PF in the flax root test shows some resemblance to that of 2:3:5-tri- 

 iodobenzoic acid (TIB) (cf. Aberg, 1953a; 1955; Aberg and Jonsson, 1955). 



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